1. Field of the Invention
This invention relates to lighting units using light emitting diodes (LEDs) and more particularly to LED based lighting units for illuminating channel letters.
2. Description of the Related Art
Recent developments in LEDs have resulted in devices that are brighter, more efficient and more reliable. LEDs are rugged, consume less power, have a relatively long life (up to 100,000 hours), operate at low voltage (7V), and are 30 to 70% more energy efficient than conventional lights, such as neon or fluorescent bulbs.
Channel letters are commonly found on the outside of buildings and are often used to advertise the name of the business. They are typically constructed of an aluminum or plastic housing that is in the shape of a letter and is approximately 5″ deep. The housing has a generally U-shaped cross-section, with the opening in the housing covered by a colored plastic translucent lens that transmits light from within the housing.
Channel letters are typically illuminated with neon or fluorescent light sources that are mounted within the channel letter housing. Neon and fluorescent lights provide a bright and continuous light source that allows the channel letters to be visible at night. However, these light sources have a relatively short life (20,000 hours), are fragile, operate at high voltage (7,000 to 15,000 volts for neon) and can consume a relatively large amount of power. Neon bulbs can also experience difficulty with cold starting, which can lead to the bulb's failure.
LEDs have more recently been used as the light source in different applications. U.S. Pat. No. 5,697,175, to Schwartz, discloses a low power illuminated sign that is particularly adapted for use with common EXIT signs over doorways. The back of each sign comprises a reflector with a series of cavities with curved surfaces. Each cavity corresponds to a letter and background area in the sign. LEDs are mounted in the center of the cavities to illuminate the letters or background area. The LEDs are provided on a separate perpendicular circuit board or on a central projection formed in the bottom of the cavities, with light from the LEDS directed outward. The letters and background area of the sign are illuminated by light reflecting forward from the curved surfaces of the cavities, so that the only visible light is from the illumination of the cavities.
The Shwartz lighting arrangement is not compatible with channel letters because the channel letter housing does not have curved surfaces to reflect light forward. Further the Shwartz arrangement can be prohibitively complex and costly for channel letters and the system provides no mechanism for dissipating heat in from the LEDs.
U.S. Pat. No. 6,042,248, to Hannah et al., discloses an LED assembly for channel letter illuminating signs having an enclosure/housing covered by a translucent lens. Each sign includes a plurality of track moldings at the base of its enclosure, with the moldings running along the longitudinal axis of the sections of the channel letter. Linear arrays of LEDs are mounted on printed circuit boards (PCBs) that are mounted in the track moldings. Each track molding can hold two PCBs in parallel with each of the PCBs arranged on a longitudinal edge, with the LEDs directed outward.
One disadvantage of the Hannah arrangement is that it is not flexible enough to be easily mounted to curved sections of channel letters. The process of mounting moldings to the channel letters can also be complicated and time consuming. This arrangement also utilizes two continuous LED linear arrays to illuminate the sections of channel letters along with a molding, which can be prohibitively complex and expensive.
LED based channel letter lighting is also available from LumiLEDs, Inc., under part numbers HLCR-KR-R0100 and HLCR-KR99-R0200, which comprises LEDs that are each mounted by insulation displacement connectors (IDC) on two inch centers. The chain of LED modules is then mounted into a bendable clip or rail, each of which are then mounted inside a channel letter to hold the LEDs in place. Power is provided by a combination of an AC/DC mother power supply and a DC/DC daughter power supply. A sensing LED is also included as a temperature and current sensor.
One disadvantage of this channel lighting arrangement is that it is difficult to install because each of the modules must be individually mounted on the wires using an IDC. They must then be mounted in the channel letter using custom installation tool. The modules do not include structures to help dissipate heat and faulty modules are difficult to remove and replace. The system uses six modules per foot and the power supply is complex and expensive. This system can be prohibitively expensive for many applications.